Polishing is a technological process, which is widely used in semiconductor industry for manufacturing, e.g., semiconductor wafers with surfaces of high planarity.
Planarized surfaces are highly desirable on shallow trench isolation layers, inter-layer dielectrics, inter-metal dielectrics, and other layers used in modem microelectronics. The polishing planarization process is important since, in order to fabricate the next level circuit, high-resolution lithographic processes must be utilized. The accuracy of a high-resolution lithographic process can only be obtained when the process is carried out on a substantially flat surface. The planarization process is therefore a crucial processing step in the fabrication of a semiconductor device.
A planarization process can be carried out by chemical-mechanical polishing, or CMP. The process has been widely used in fabricating semiconductor devices of various types, primarily for polishing the front or device surface of a semiconductor wafer for achieving planarization and for preparation for the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible.
A CMP process can be performed, e.g. by using a rotating polishing platen with a polishing pad in combination with a pneumatically actuated rotating head, holding the wafer.
A wafer can be polished in a CMP apparatus by being placed on a carrier, assembled on the rotating head, and pressed down, under controlled pressures, onto a polishing pad, which is attached on the platen and covered with a polishing slurry, e.g. of colloidal silica or alumina. A polishing pad used on a rotating platen is typically constructed of two layers overlying a platen with a resilient (elastic) layer as an outer layer of the pad. The layers are typically made of polymeric materials. A polishing pad may be made larger than a wafer while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface on the wafer. The pad may have an orbital rotation. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile on the wafer surface. The axes of rotation of the wafer and pad are deliberately not collinear, though parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity, and concentration of the slurry.
Both the CMP and traditional mechanical abrasion polishing processes have a difficulty of their process control. In particular, end point detection, or exact determination of a moment at which the polishing process has to be stopped, has been a problem for the industry. The CMP process is frequently carried out without a clear signal about when the process is completed, just by using only empirical polishing rates and timed polish instead. Since the calculation of required polish time based on empirical polishing rates is frequently inaccurate, the empirical method fails frequently, resulting in serious yield drops and waste of the expensive wafers with devices.
Therefore, process control and end point detection are the important issues for automation of the aforementioned processes.
Known in the art are end point detection methods utilizing optical, acoustical, electrical, and mechanical measurements.
Optical methods of end point detection in polishing are based on surface reflectivity, light transmission, and interferometry measurements, e.g., by means of a laser beam. An example of such method is described by Koos et.al., in U.S. Pat. No. 5,413,941. Here the wafer being polished is faced against a mirror as it comes off-platen. Laser light is passed transversely between the wafer surface and the mirror, causing multiple reflections between mirror and wafer. A detector at the opposite side records the linear intensity signature of the wafer surface from the exit beam. This signal is a direct measure of the degree of surface planarity and as such provides a monitor of the planarization process.
Optical methods, however, may not be applicable to chemical mechanical polishing in some cases, since the CMP can involve the use of nontransparent media such as polishing slurry. Therefore for CMP process control other methods of end point detection are required.
Examples of CMP process control method based on measurement of the running motor current in order to detect the variations of the motor torque, related to the variations of mechanical parameters in the zone of contact of the rotating pad with the surface being treated (such as friction coefficient), are described, e.g., in U.S. Pat. Nos. 5,948,706, 5,830,041, 5,308,436. Another method and apparatus (see, e.g., U.S. Pat. No. 5,738,562) for end-point detection during CMP has been developed based on the measurements of the variations of translational (lateral) motion of the polishing platen, related to the variations of the friction coefficient of different film materials. Both these methods are based on indirect measurement techniques, used for very approximate evaluation of the friction variations, cannot produce accurate measurements of the friction coefficient and thus, are not used for the practical CMP process control.
The CMP processes typically involve polishing of one external layer on the wafer surface till the next layer is exposed. Usually these layers comprise various combinations of materials, like metals, semiconductors and dielectrics. When these layers have different electrical characteristics, like electrical conductivity, dielectric constant, etc., the measurements of these characteristics may be used for end point detection.
For instance, one of these methods refers to the measurement of electrical resistance of a special test structure formed on the wafer surface (e.g., U.S. Pat. No. 6,015,754). Implementation of this method requires electrical contacts to be provided to the wafer surface at certain locations and a special test pattern to be created on the surface before processing, which complicates the process and is impractical.
Another method and apparatus is based on the measurements of electrical resistance of the wafer structure. It includes a pair of electrical contacts that connect the surface of a material being polished to the measurement circuitry, said contacts present two openings in the polishing pad filled with a conductive material such as conductive epoxy (e.g., U.S. Pat. No. 5,242,524).
Similarly, in yet another method and apparatus for end-point detection during CMP of a dielectric layer, which uses the measurements of electrical capacitance of the wafer structure, a pair of electrodes, central and a surrounding guard electrode, forms a thickness detection region, that is, a capacitance probe. Said electrodes embedded within and electrically isolated from the polishing table and electrically contacting the wafer surface via openings in the polishing pad filled with a conductive material or conductive slurry (e.g., U.S. Pat. No. 5,337,015).
Another group of methods and apparatus for end-point detection during CMP is based on the measurements of sound waves or acoustic noise, generated in the interface between the wafer and polishing material, and detected with a microphone (e.g., U.S. Pat. Nos. 5,439,551, 5,222,329).
All these methods and apparatuses are based on the fact that when one layer is removed and the next layer is exposed by CMP, this transition is characterized by noticeable changes in the amplitude and spectrum of an acoustic signal. The acoustic signals are detected with the use of an acoustic signal receiver (such as a microphone) located in the vicinity, but outside of the polishing zone of a wafer-pad contact. The signal is then recorded in a manner known in the art and used for a process analysis.
It is understood that in CMP the main source of acoustic signal, which contains valuable information, is a zone of direct contact of the polishing pad with an object being polished such a semiconductor wafer. When the acoustic receiver is located outside the contact zone and separated from the signal source by an air gap, the recorded acoustic signal has a low level and contains a lot of environmental noise. The recorded signal may be especially noisy, when there is no signal filtering done to filter out the noise. Therefore a signal-to-noise ratio is very low, which reduces the accuracy of process control.
Yet another method and apparatus for end-point detection during CMP are based on the measurements and analysis of low-frequency vibrations (typically, less than 50 kHz), caused by interactions of the polishing pad and semiconductor wafer in the course of polishing (e.g., U.S. Pat. No. 5,876,265).
Both methods, acoustical and vibrational, use relatively low-frequency measurements in a bandwidth of just several kilohertz, and thus do not provide high accuracy and resolution in micro-scale process detection required for thin-film polishing and planarization. Also, there is significant influence of environmental noise and vibrations having similar bandwidth.
Thus, in addition to disadvantages mentioned above, each existing mechanical, acoustical, and electrical method for controlling CMP processes is applicable to certain specific conditions of treatment and has a limited application, if any, under other conditions. Another disadvantage of all existing methods is lack of uniformity of CMP control. Moreover, when the CMP is used for planarization within the same layer on the wafer, none of the known methods of polishing process control provides reasonable effectiveness and reliability.